This may be harsh, so I will apologize in advance.....
An old SIer often said that his wife described being a BS as being raped. That the AP held her down while the WH raped her. That has ALWAYS resonated with me. I'm in year 4 and I still feel like that. I still feel that EVERY time I had sex with my WH prior to dday was sex without informed consent. This perspective can get touchy, which I understand. Not everyone shares that sentiment, but I and the wife of this old SIer are 100% on the same page (and IIRC, I believe his As were EAs more than PAs - not that it matters, really).
I don't say this to make you feel worse than I suspect you already do. I say this to try and help you see just how damaging living in the dark is. That once the bubble of an A has popped, continuing to withhold such VITAL info from another BS is wayward brain, stinkin thinking, fill it in however you like, but it continues your role as an accomplice to the harm of another human being.
So, you tell the OBS. You do it for all the reasons already stated. It's the right thing and it helps you. If she already knows her WH is cheating, then you've not changed the status quo. If she does not know, then you have helped her learn of her actual reality.
Moreover, telling her shows YOU have taken responsibility for YOUR actions vis-a-vis another M.
I'll echo others that ideally, it would be your BH that tells the OBS. But if he is not willing to do so, then it's up to you. Who told me or why is far less important than that I found out what my WH had been up to and the level of deceit to which he is capable. FWIW, it really bothers me that my WH seems to have zero sense of remorse or empathy for the harm he caused the OBS in my situation.
Telling yourself that it's not your place to tell the OBS is far more likely simply protecting yourself from guilt, shame, discomfort, etc., as well as the same for your AP. Of COURSE no one "wants" to make such a call. No one WANTS to call someone with bad news, esp if the the caller was a participant.
If your child stole from another, what would you have them do? Would you have them ignore it? I would have my kid go and confess to the owner, return what was stolen, and apologize. Unfortunately, there is so much stolen by an A that cannot be "returned", the ONE thing you have absolute power to return to this OBS is her reality - and IMHO, you absolutely owe her that.
Accountability (and integrity) is tough stuff, and it's also absolutely necessary.
ETA:
You can't leave it behind if it's still an unopened surprise for the OBS. You have to take responsibility for the people you hurt.
Amen, and think of this: Say you and your BH are able to do the work and reconcile w/o telling the OBS. Just try for a moment to imagine what can happen if 1, 3, 5, 10, 20 years down the road the OBS finds out and contacts your BH? Can you imagine how much that can reset the clocks to zero? Why would anyone want that kind of "surprise" hanging out there?
[This message edited by gmc94 at 5:59 PM, April 8th, 2021 (Thursday)]